No. 2.] THE HARD PARTS OF THE MAMMALIA. 193 
face of the fore arm (radius) have gradually, under the influence 
of countless blows, impressed themselves into the latter. On 
the contrary, the surface above the weaker lines between the 
bones not having been subject to the impact of the blows, and 
influenced by inertia and gravity, remains to fill the grooves, 
and to form the keels which we observe. 
We now consider the development of the keels and grooves 
which appear at 
the articulation of 
the metapodials 
Fic. 33. 
Figure 33. — Hind foot of primitive cameloid (Poébrotherium labiatum), showing 
grooved astragalus and first toe-bones, without keel, in front at lower end; from Col- 
orado. 
Figure 34. — Hind foot of three-toed horse (Protohippus sejunctus), from Colo- 
rado, showing grooved astragalus, and trace of keel on front of lower end of first bone 
of middle toe. 
Figure 35.— United first bones of two middle toes of deer-antelope (Cosoryx 
furcatus), showing extension of keel on front of lower end; from Miocene of Ne- 
braska. 
